The production of foamable particles of styrenic polymers, in particular polystyrene, is well-known in the art. It consists in incorporating into the polymer, during the polymerization step, or at a later time, a foaming agent boiling at a lower temperature than the polymer softening temperature.
When the particles which contain the foaming agent are heated, the foaming agent evaporates and generates a very large number of closed hollows, viz., cells, in the polymer.
In order to produce low-density finished articles or blocks, these particles are charged into the hollow of a mould which defines the shape of the desired finished article, and are then heated up to a higher temperature than the boiling point of the foaming agent, and than the softening point of the polymeric material. During this heating stage, the particles are foamed and, owing to the limited available room, get sintered, forming a body having the shape, and the dimensions, of the hollow of the mould used.
The particles can be directly charged to the mould, or they can be pre-formed and, before being charged to the mould, they may be submitted to ageing, for time periods of approximately 15-30 hours.
After the moulding, the resulting shaped article is allowed to cool inside the mould for a period of time long enough to prevent the moulded article from undergoing deformations after being de-moulded.
Inasmuch as the foamed plastic material is a very good heat insulator, in order to cool the formed body, relatively long residence times inside the mould are required.
Therefore, the cooling time represents a large portion of the moulding cycle and enormously reduces the mould throughput during a given time period.
In order to reduce the residence time inside the mould, and simultaneously obtain a good sinterization outcome, in the past the addition was proposed of small amounts of halogenated organic compounds, such as bromo-, chloro- or chloro-bromo-derivatives, to be added to the styrenic polymer at polymerization time.
Some examples of these organic compounds, added to styrene during the polymerization step, are reported in U.S. Pat. Nos. 4,169,193; 4,172,928; and 4,200,696.
These organic products make it possible the residence time inside the mould to be reduced to an even considerable extent, but suffer from several drawbacks which limit the use thereof. A first drawback is due to the development of unpleasant smells during the pre-foaming step. Furthermore, after the moulding high shrinkage rates are observed, consequently moulded bodies with pre-established dimensions cannot be obtained.
Furthermore, the use of halogenated organic products may cause toxicity problems when the material is used in food packaging applications. As well-known, the various national Law regulations suggest that halogenated products should be eliminated from the formulations of foamable polystyrenes.
In European patent publication No. 0 046 494, coating the particles of styrenic polymers with a hydroxycarboxy acid ester or a carboxy acid ester of an alkoxy alcohol is proposed. Such a coating imparts to the material very short moulding times, and very good sintering ability; unfortunately, it displays the drawback that large amounts of additives have to be added to the particles in order to have acceptable results, with consequent problems of caking of the particles inside the containers, and of pollution of process water during the moulding operation.